State-of-the-art propellant formulations, at their most basic level, are composed of an oxidizer and a fuel. The combustion reaction undergone by these two materials provides the energy necessary to propel the rocket or missile. Since the oxidizer/fuel combination must sustain the stresses of handling, aging, storage and use, it is typically compounded in a formula consisting of binder, plasticizer and various solid ingredients. Ideally, all the components in the formulation act as either oxidizers or fuels, contributing to the energy necessary for maximum propulsion performance, although in practice, certain necessary ingredients such as stabilizers and bum rate catalysts/modifiers, have little or no energy to impart to the reaction.
The performance of the propellant is directly proportional to the enthalpy release of the oxidizer and fuel ingredients as they undergo combustion, and inversely proportional to the molecular weight of the gases produced in the combustion reaction. In practice, some tradeoffs are necessary to gain the best performance from available ingredients and formulations. Aluminum, for instance, is a fuel whose combustion products are relatively high in molecular weight, and are in most cases, not gases at all, but solids. However, the enthalpy release by the combustion of aluminum is so great in proportion to anything, which would otherwise be available as a fuel ingredient, that the metal is commonly used as a fuel in high-performance tactical and strategic rocket motor applications. Another material commonly utilized, despite some drawbacks, is the oxidizer ammonium perchlorate. This material has a high negative enthalpy of formation, limiting its energy release upon combustion, and, in addition, it produces hydrogen chloride upon combustion, a relatively high-molecular-weight toxic gas. However, ammonium perchlorate is inexpensive, easy to formulate, has very tractable ballistics and favorable burn characteristics, and so, despite its limitations, it is the state-of-the-art oxidizer for most solid propellant rocket motor formulations.
Ammonium dinitramide (ADN) is a very powerful organic oxidizer that can replace ammonium perchlorate (AP) in propellant compositions. ADN has the following formula: Calculations have shown that, when incorporated in propellant formulations, the ADN propellant can achieve performance equal to or higher than that of the conventional hydroxyl-terminated polybutadiene (HTPB)/AP propellant. Most desirably, ADN propellants do not produce toxic hydrogen chloride (HCl) in the exhaust. In addition, the use of ADN in propellant formulations greatly minimizes the secondary smoke problem caused primarily by the nucleation of HCl. Because of their environmentally friendly characteristics and demonstrated low toxicity of their exhaust products to humans, ADN propellants are highly desirable. In recent years, investigators have been designing propellant formulations that try to embody the advantages of ADN as a solid oxidizer.
The need to have missiles fly farther, higher and faster and to carry heavier payloads is a constant tactical and strategic factor. Higher performance is always needed. In volume-limited systems, this performance can only come about by increases in the quantity, density or energy of the propellant formulation, by decreases in the weight of the inert hardware and the airframe, and by operating at higher pressures. A new requirement has come to light in recent years: that the formula and its combustion products be nondegrading to the environment. In the light of these requirements, state-of-the-art propellant formulations utilizing conventional binders, ammonium perchlorate and aluminum have been developed and refined to the maximum extent possible and these compositions will necessarily begin to fall behind in performance compared to newer developments. In addition, the political and environmental concerns with the toxic and corrosive hydrogen chloride present in the exhaust of rockets utilizing these formulations will result in demands to replace such formulations with more innocuous compositions.
ADN material synthesized at Bofors, Sweden has developed a new synthesis route for ADN. This new synthesis technique is the result of collaboration between the scientists at Bofors and at the National Defense Research Establishment of Sweden. This new synthesis route, illustrated in FIG. 1, can drastically reduce the cost of ADN manufacturing in comparison to the methods previously used.
U.S. Pat. No. 6,074,581 issued to Wood et al. on Jun. 13, 2000, incorporated herein by reference, discloses a method of producing ADN prills using molten ADN and mineral oil emulsion technology. The ADN prills produced using the process of the '581 patent have a particle size of 20-350 μm. U.S. Pat. No. 6,135,746 issued to Wood et al. on Oct. 24, 2000, incorporated herein by reference, discloses an apparatus for producing ADN prills using molten ADN and mineral oil emulsion technology. The ADN prills produced using the process of the '746 patent have a particle size of about 20 to about 350 μm.
Another method of producing ADN prills involves the use a prilling tower in which ADN particles pass through a hot air zone and are fused into rounded spheres. During this prilling process, 0.5% urea is added as the thermal stabilizer and 0.2% carbosil is added to prevent moisture pick-up. These ADN prills have an average particle size of about 100 to about 200 μm.